For ophthalmic imaging systems that involve scanning microscopy, the detail and resolution of the resultant image is directly dependent on both the size of the illuminating light beam's focal spot, and the quality of the optics in the imaging system. As for the size of the focal spot, particularly when imaging an eye's retinal tissue, it is desirable that the spot size be as small as possible. Due to the structural size of retinal tissues, this means that spot sizes of around only two microns in diameter may be useful. As for the quality of the optical system, this factor is typically evaluated by what is known as the Point Spread Function (PSF). More specifically, the PSF pertains to an intensity distribution that describes the response of an imaging system to a point source of light or to a point object. The degree of spreading (i.e. blurring) of the point object is then a measure for the quality of the imaging system.
Not surprisingly, the PSF of an imaging system can be adversely affected by several factors. In particular, image resolution with a PSF will be limited by such factors as imperfections in the lenses of the optical system, a misalignment of the lenses and, in the specific case of ophthalmic imaging applications, aberrations introduced by the eye itself. On this last point, it is to be appreciated that when imaging the retina, the anterior components of the eye (i.e. the cornea and the lens of the eye), as well as the retina need to be considered along with the optical components of the imaging system. Due to diffraction, however, there is still a fundamental maximum to the resolution that can be attained by an imaging system. Specifically, an optical (imaging) system having the ability to produce images with an angular resolution that is as good as the instrument's theoretical limit is said to be “diffraction limited.” Thus, for ophthalmic imaging systems, the objective is to attain a Diffraction Limited Point Spread Function (DL-PSF).
A technical concept associated with the PSF of an imaging system that operates close to its diffraction limit is the “Strehl Ratio.” By definition, the Strehl Ratio is the ratio of an observed peak intensity compared with the theoretical maximum peak intensity of a perfect imaging system working at the diffraction limit. Stated differently, the Strehl Ratio can be defined as the best focus of the imaging system. Importantly, the Strehl Ratio for a given optical (imaging) system is determinable, and variations therefrom are observable.
In the context of an ophthalmic imaging system, it is known that when anatomically introduced optical aberrations are introduced into the light beam of an optical system they can be measured. Further, it is known that such aberrations can be compensated for. For example, U.S. patent application Ser. No. 12/204,674 for an invention entitled “Custom Phase Plate,” which is assigned to the same assignee as the present invention, discloses a customized phase plate for removing optical aberrations from a light beam when they have been introduced by the retina and the anterior components of an eye. Optical aberrations that are anatomically introduced, however, are both static and dynamic. This is in contrast with an optical (imaging) system that remains substantially static during an imaging procedure.
In light of the above, it is an object of the present invention to provide a system and method to compensate for static and dynamic aberrations that are introduced into an imaging light beam during an imaging procedure. Another object of the present invention is to provide and maintain a substantially DL-PSF for a high quality optical system during an imaging procedure. Still another object of the present invention is to provide a system and method for optimizing the PSF of an imaging system that is easy to use, is relatively simple to manufacture and is comparatively cost effective.